In a typical cellular communications system, wireless user equipment units (UEs), for example, mobile phones, communicate via a radio access network to one or more core networks. A radio access network covers a geographical area which is divided into cells, with each cell area being served by a radio base station. Several base stations are connected, typically via land lines, to a control node known as a radio network controller (RNC). Such a control node supervises and coordinates various activities of the several radio base stations which are connected to it. The radio network controllers are typically connected to one or more core networks. One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation (3G) system and UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units. Fourth generation systems are evolving towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network.
In many radio access networks the radio base station is a concentrated node with most of its components being located at a concentrated site. However, a radio base station can also be configured with a more distributed architecture. For example, a distributed radio base station can take the form of one or more radio equipment (RE) portions that are linked to one or more radio equipment control (REC) portions over an internal interface. One example of an internal interface of a radio base station which links a radio equipment portion of the radio base station to a radio equipment control portion of the base station is the Common Public Radio Interface (CPRI). The Common Public Radio Interface (CPRI) is described in Common Public Radio Interface (CPRI) Interface Specification Version 5.0 (2011).
The Common Public Radio Interface (CPRI) is an industry cooperation aimed at defining a publicly available specification for the key internal interface of radio base stations between radio equipment control (REC) and radio equipment (RE), thereby allowing base station manufacturers to share a common protocol and more easily adapt platforms from one customer to another. In essence, a radio base station is decomposed into two separate blocks, known as REC and RE. The REC provides access to a UMTS network, for example, via the lub interface, whereas the RE serves as the air interface to user equipment, known as Uu in a UMTS network. The REC generally comprises the radio functions of the digital baseband domain, whereas the RE contains analogue radio frequency functions.
The CPRI interface supports several types of information flow, notably user plane information in the form of in-phase and quadrature (IQ) modulation data (the digital baseband signals) and control and management (C & M) information which is exchanged between the control and management entities within a REC and a RE. The CPRI has a basic frame structure for carrying a control word and an IQ data block. The IQ data of different antenna carriers are multiplexed by a time division multiplexing scheme onto an electrical or optical transmission line. The control and management data are either sent as inband protocol or by layer 3 protocols that reside on top of appropriate layer 2 protocols. C&M data are time-multiplexed with synchronisation data and the IQ data over the CPRI. Two different layer 2 protocols for C&M data are supported by CPRI. These are Ethernet and High-Level Datalink Control (HDLC). A vendor-specific channel may also be supported.
HDLC is a protocol developed by the International Organisation for Standardisation and falls under ISO standards ISO 3309 and ISO 4335 HDLC is sometimes referred to as the slow C&M channel in CPRI (Ethernet being referred to as the fast channel) HDLC can support several data rates and operates to provide a reliable communications path between nodes with acknowledged data transfer. Each piece of data is encapsulated in an HDLC frame with a trailer and a header (address and control field). The trailer has a cyclic redundancy check (CRC) and the frames are separated by a flag sequence.
The functional split between the REC and RE allows the RE to be positioned close to an associated antenna. This reduces the distance which the associated signals have to travel before they are received by the RE, thereby negating the need for tower-mounted amplifiers and antenna system controllers. The link between the RE and REC is generally optical, allowing the link length to be much greater when compared with wired coaxial systems. Therefore, the distance between the RE and REC can be up to 40 Km, thereby increasing the flexibility of deployment of RE-s within the network when utilising CPRI. One REC may be linked to two or more RE-s or one RE may be linked to multiple REC-s in a chain topology with each REC being configured to forward data to other REC-s in the chain. The CPRI daisy chain configuration consists of multiple REC-s and multiple RE-s. The REC chain comprises the baseband units and the RE chain comprises the remote radio head units.
Many of the functions which an REC has to perform, which include channel coding/encoding, spreading/despreading, frame and time slot generation, for example, may be realised by a proprietary digital signal processing device. Two examples of such DSP devices which support the CPRI are the Freescale B4860 and the Freescale MSC 8157 Broadband Wireless, Access Six Core DSP which is described in Freescale Semiconductor Data Sheet MSC8157E, November 2011. This Digital Signal Processor includes (inter alia) a CPRI unit Each CPRI unit may support two CPRI lanes, each lane typically comprising a framer module and a direct memory access (DMA) module for transferring receive and transmit data to and from a system memory.
The CPRI protocol defines an HDLC stream in a way such that only the REC which has full knowledge of the topology is permitted to manage the stream (that is the end-point REC). A similar situation applies to the RE chain where only the end-point RE may handle the stream. This has the drawback of (despite all components in the chain being able to read the HDLC packets) only the endpoint REC or endpoint RE being able to write. This restriction does not allow the networking REC-s or RE-s to report on their status or to deliver control messages.